1. Field of the Invention
The present invention relates to an apparatus for heat treatment employed in the manufacturing process of semiconductors, and, more specifically, it relates to an apparatus for heat treatment employed for heating or cooling a semiconductor wafer or a glass substrate (hereinafter generically referred to as a substrate) using a hot plate or a cooling plate.
2. Description of the Prior Art
A conventional apparatus for heat treatment of particular interest to the present invention is disclosed in, for example, Japanese Patent Laying-Open Gazette No. 37848/1982. Present FIGS. 1A to 1D show the structure of the heat treatment apparatus shown in the foregoing document. FIG. 1A is a perspective view of the heat treatment apparatus and FIG. 1B is a cross sectional view of the portion shown by the line IB--IB in FIG. 1A. Referring to FIGS. 1A and 1B, the heat treatment apparatus comprises a heating section HS, a heat transmitting section HT formed thereon, and a wafer placing section HP formed on the heat transmitting section HT. The heating section HS comprises heater blocks HB. The upper surface of the wafer placing section constitutes a surface of heat radiation section HPS. The surface of heat radiation section HPS is formed so as to enable effective heat radiation. A wafer W which is to be subjected to the heat treatment is placed on the surface of heat radiation section HPS with a spacer S interposed therebetween. The spacer S has a rod like shape.
FIGS. 1C and 1D show variations of the spacer S. The spacer S may be a lattice (FIG. 1C). The spacer S alternatively, may comprise three claw portions (FIG. 1D).
According to the above mentioned gazette, the conventional heat treatment apparatus has the following advantages. Namely, since the wafer is spaced above the heat radiation section of by a spacer, the wafer is heated by the heat radiation from the surface of the heat radiation section. Therefore, the wafer is heated with the temperature distribution being uniform. In addition, the heating ability is not very much degraded so long as the spacing between the surface of the heat radiation and the wafer is about 1 mm.
The conventional apparatus for the heat treatment of a substrate has the above-described advantages. However, the spacing d (in FIG. 1B) between the heating plate and the substrate has influences on the heating time when the substrate is heated or cooled to a prescribed temperature by the heat radiation. When the spacing d is about 1 mm, the influence is large.
FIG. 2 is a graph showing the relation between the temperature of the substrate and the time required for heating (hereinafter referred to as the heating characteristics) when the set temperature is 120.degree. C. FIG. 3 is a graph showing the relation between the time required for cooling and the temperature of the substrate (hereinafter referred to as the cooling characteristics) when the set temperature is 25.degree. C. The reference character d indicates the spacing between the heating plate and the substrate. The smaller the spacing d becomes, the faster the substrate reaches the set temperature.
In view of these graphs, the spacing d should be as small as possible, so long as it does not affect the substrate. By doing so, the rate for treating the substrate can be increased.
However, as described above, the conventional spacer has the shape of rod, lattice or a claw. Therefore, the spacer must be machined with higher precision in order to keep the spacing minute. For this reason, it was difficult in the conventional apparatus for the heat treatment of the substrate to keep the spacing small. If the spacer is machined with high precision in order to keep the spacing small, the cost of the substrate treatment apparatus becomes higher.
Meanwhile, the temperature distribution on the substrate should preferably be uniform. Therefore, the temperature difference between the portion where the substrate and the spacer are in contact with each other and the portion where they are apart from each other should be small. In order to attain the above described condition, the contact area of the spacer and the substrate should be as small as possible. When a conventional rod like or lattice spacer is employed, the contact area cannot be made small.